U.S. patent application number 12/528486 was filed with the patent office on 2010-04-08 for refrigerant system with expander speed control.
Invention is credited to Alexander Lifson, Michael F. Taras.
Application Number | 20100083678 12/528486 |
Document ID | / |
Family ID | 39831221 |
Filed Date | 2010-04-08 |
United States Patent
Application |
20100083678 |
Kind Code |
A1 |
Lifson; Alexander ; et
al. |
April 8, 2010 |
REFRIGERANT SYSTEM WITH EXPANDER SPEED CONTROL
Abstract
A refrigerant system utilizes an expander to expand refrigerant
and to drive or assist in driving an associated compressor. By
varying the compressor load, the speed of the expander can be
adjusted to achieve the desired thermodynamic characteristics of
the expanding refrigerant and enhance expander operation.
Inventors: |
Lifson; Alexander; (Manlius,
NY) ; Taras; Michael F.; (Fayetteville, NY) |
Correspondence
Address: |
CARLSON, GASKEY & OLDS, P.C.
400 WEST MAPLE ROAD, SUITE 350
BIRMINGHAM
MI
48009
US
|
Family ID: |
39831221 |
Appl. No.: |
12/528486 |
Filed: |
April 10, 2007 |
PCT Filed: |
April 10, 2007 |
PCT NO: |
PCT/US07/66278 |
371 Date: |
August 25, 2009 |
Current U.S.
Class: |
62/115 ;
62/510 |
Current CPC
Class: |
F25B 2400/13 20130101;
F25B 2400/14 20130101; F25B 2600/0261 20130101; F25B 1/10 20130101;
F25B 9/06 20130101; F25B 9/004 20130101 |
Class at
Publication: |
62/115 ;
62/510 |
International
Class: |
F25B 1/00 20060101
F25B001/00; F25B 1/10 20060101 F25B001/10 |
Claims
1. A refrigerant system comprising: a main compressor to compress
refrigerant and to circulate this refrigerant throughout said
refrigerant system; a secondary compressor, said secondary
compressor at least partially driven by an expander, where at least
a portion of the refrigerant circulating throughout said
refrigerant system is expanded in said expander from a higher
pressure to a lower pressure; and a control for determining desired
characteristics of an expansion process, and said control being
operable to control a load on the expander provided by said
secondary compressor to achieve said desired characteristic of said
expansion process.
2. The refrigerant system as set forth in claim 1, wherein said
higher pressure is associated with a system high-side heat
exchanger.
3. The refrigerant system as set forth in claim 1, wherein said
lower pressure is associated with a system low-side heat
exchanger.
4. The refrigerant system as set forth in claim 1, wherein said
higher pressure is associated with an intermediate system
pressure.
5. The refrigerant system as set forth in claim 1, wherein said
lower pressure is associated with an intermediate system
pressure.
6. The refrigerant system as set forth in claim 1, wherein said
secondary compressor compresses at least a portion of refrigerant
in said refrigerant system.
7. The refrigerant system as set forth in claim 1, wherein the
speed of said expander is controlled by the load on said secondary
compressor.
8. The refrigerant system as set forth in claim 7, wherein a valve
is controlled to control the load on said secondary compressor.
9. The refrigerant system as set forth in claim 8, wherein said
valve is an unloader bypass valve.
10. The refrigerant system as set forth in claim 8, wherein said
valve is a suction throttling valve.
11. The refrigerant system as set forth in claim 8, wherein said
valve is controlled by pulse width modulation technique to achieve
said desired load.
12. The refrigerant system as set forth in claim 1, wherein said
secondary compressor compresses fluid other than the refrigerant
circulating throughout said refrigerant system.
13. The refrigerant system is set forth in claim 1, wherein the
amount of fluid compressed and passed downstream by said secondary
compressor is changed to change a speed of the expander.
14. The refrigerant system is set forth in claim 1, wherein the
refrigerant passing through said expander passes at least in part
to said main compressor.
15. A method for operating a refrigerant system comprising the
steps of: (1) providing a main compressor to compress refrigerant
and to circulate this refrigerant throughout said refrigerant
system; (2) providing a secondary compressor, said secondary
compressor at least partially driven by an expander, where at least
a portion of the refrigerant circulating throughout said
refrigerant system is expanded in said expander from a higher
pressure to a lower pressure; and (3) determining desired
characteristics of the expansion process, and changing a load on
the expander provided by said secondary compressor to achieve said
desired characteristic of said expansion process.
16. The method as set forth in claim 15, wherein the speed of said
expander is controlled by the load on said secondary
compressor.
17. The method as set forth in claim 15, wherein a valve is
controlled to control the load on said secondary compressor.
18. The method as set forth in claim 15, wherein said secondary
compressor compresses fluid other than the refrigerant circulating
throughout said refrigerant system.
19. The method as set forth in claim 22, wherein the amount of
fluid compressed and passed downstream by said secondary compressor
is changed to change a speed of the expander.
20. The method as set forth in claim 15, wherein the refrigerant
passing through said expander passes at least in part to said main
compressor.
Description
BACKGROUND OF THE INVENTION
[0001] Refrigerant systems are known to utilize refrigerant
circulating throughout a closed-loop circuit to condition a
secondary fluid. Typically, a refrigerant system includes a
compressor for compressing the refrigerant, and delivering the
refrigerant to a downstream heat exchanger. Refrigerant from that
downstream heat exchanger passes through an expansion device, and
then to an evaporator. In traditional refrigerant systems, the
expansion device is a fixed area restriction or a valve that may be
controlled such that the amount of expansion is tailored to achieve
desired characteristics in operation of the refrigerant system.
[0002] In some advanced refrigerant systems, the work which is
available from the expansion process of the refrigerant is utilized
to drive or assist in driving at least one component within the
refrigerant system.
[0003] In one known refrigerant system configuration, a secondary
compressor operates in parallel with a main compressor. This
secondary compressor compresses a portion of the refrigerant
circulated throughout the refrigerant system. The secondary
compressor is driven by the expander, with the expander operating
much like a turbine, to receive the compressed refrigerant, and
expand that refrigerant to a lower pressure and temperature. The
work from this expansion process is utilized to drive the secondary
compressor. This known combination of a compressor and an expander,
located on the same shaft, is called an expresser. The use of the
expresser is known in the industry, where the expander drives or
assists in driving the corresponding compressor. The refrigerant
exiting a heat rejection heat exchanger enters the expander, and
then is expanded to a lower pressure and temperature. A two-phase
flow exiting the expander enters the evaporator. The work extracted
from the expansion process in the expander is used to drive the
secondary compressor that is quite often located on the same shaft
as the expander. In addition to extracting useful work from the
expansion process, the refrigerant passing through the expander
acquires a higher cooling thermodynamic potential, as it expands
through the expander, since it follows a more efficient isentropic
process. The use of the expresser technology is especially expected
to grow in CO.sub.2 applications, where the potential for the
expansion energy recovery is higher than for the conventional
refrigerants.
[0004] One of the disadvantages of positioning the expander and the
associated compressor into a closely coupled mechanical engagement,
such as locating them on the same shaft, is that the expander speed
is not actively controlled. In other words, the expander will
settle at a speed at which the power extracted by the expander from
the refrigerant expansion process is roughly equal to and is
balanced by the power delivered to the compressor. Since the
expander speed cannot be actively controlled, the expansion process
through the expander is typically not optimal. If the expansion
process is not optimal, then the amount of refrigerant delivered to
the evaporator, and its thermodynamic state, cannot be precisely
controlled. If a delivered amount of refrigerant cannot be
adjusted, it may result, for instance, in less than optimal gas
cooler pressure, in transcritical applications, and/or undesirable
conditions at the compressor entrance.
[0005] In other words, to optimize the expansion process for given
operating and environmental conditions, such as gas cooler
pressure, suction superheat, etc., flexibility in varying the
expander speed must be provided. One way to enhance the control of
the expander is to install an expansion valve that is located in
series with the expander. However, the expansion valve would
reduce/limit the amount of the work extracted from the expansion
process by the expander. This reduction would occur, as part of the
expansion process would take place in the expansion valve, and not
in the expander. Therefore, a need exists to optimize the expresser
operation.
SUMMARY OF THE INVENTION
[0006] In this invention, the expansion process in the expander is
controlled by adjusting the speed of the expander. The higher the
expander speed, the more refrigerant can be passed through the
expander. Similarly, the lower the expander speed, the less
refrigerant passes through the expander. The expander speed of the
expresser (a mechanically coupled compressor-expender
configuration) is adjusted by changing the load on the compressor
component of the expresser. Compressor unloading can be
accomplished by using various unloading techniques such as, for
example, moving a slide valve of a screw compressor, opening a
bypass port of the scroll compressor, using suction cutoff of a
reciprocating compressor, installing a suction modulation valve, or
utilizing any other known techniques to reduce the compressor load.
This compressor load reduction causes the expander speed to
increase.
[0007] Similarly, loading the associated compressor component of
the expresser results in a speed decrease of the expander component
of the expresser. Therefore, by utilizing the proper amount of
compressor unloading we can very the expresser speed and thus
optimize the expansion process. This is true since the expander
speed varies along with the expresser speed, as both the compressor
and expander are closely mechanically coupled, such as located on
the same shaft. An ability to change the expander speed is similar
to adjusting the amount of flow by using a variable restriction
expansion device, such as an electronic expansion valve, in
comparison to inefficient fixed cross-sectional area expansion
device, such as a capillary tube or orifice.
[0008] These and other features of the present invention can be
best understood from the following specification and drawings, the
following of which is a brief description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic view of a refrigerant system
incorporating the present invention.
[0010] FIG. 2 is a view of another schematic.
[0011] FIG. 3 is a view of another schematic.
[0012] FIG. 4 is a view of another schematic.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] A refrigerant system 20 is illustrated in FIG. 1. A main
compressor 22 compresses a refrigerant received from a main suction
line 24. As shown, a secondary suction line 26 delivers a portion
of the refrigerant flow through a secondary compressor 28.
Refrigerant compressed by the secondary compressor 28 is delivered
through a secondary discharge line 30 to a main discharge line 46,
positioned on a high side of the refrigerant system 20, to be
combined with the refrigerant delivered from the main compressor
22. The combined refrigerant flow passes through a heat rejection
heat exchanger 32, where the heat is removed from the refrigerant
by a secondary fluid typically delivered to an ambient environment.
The heat rejection heat exchanger 32 is called a condenser, if the
refrigerant passes through the thermodynamic states within the heat
exchanger 32 that are below the critical point, or a gas cooler, if
the refrigerant passes through the thermodynamic states within the
heat exchanger 32 that are above the critical point.
[0014] Downstream of the condenser 32, an expansion process, to a
lower pressure and temperature, occurs in an expander 34. As known,
the expander 34 takes the compressed refrigerant from the heat
rejection heat exchanger (a subcritical condenser or a
supercritical gas cooler) 32, and utilizes energy from that
compressed refrigerant to drive the expander, while the compressed
refrigerant is "isentropically" expanded to a lower pressure and
temperature. A shaft 36 (alternatively a generator) is driven by
the expander 34, and this shaft (or power from the generator) in
turn drives the secondary compressor 28. Such systems are known as
"expressers."
[0015] A heat exchanger, or an evaporator, 38 is positioned
downstream of the expander 34. The evaporator 38 is located on a
lower pressure side of the refrigerant system 20, and heat is
transferred to the refrigerant in the evaporator 38 from a
secondary fluid to be delivered to a climate-controlled space.
Refrigerant passes from the expander 34, through the evaporator 38,
and back into the suction line 24 to return to the compressors 22
and 28. The refrigerant system 20, as described to this point, is
as known in the art. Obviously, the basic refrigerant system 20 may
have additional features or enhancement options. All these
variations in refrigerant system configurations are within the
scope and can equally benefit from the invention.
[0016] A control 50 for the refrigerant system 20 operates
components such as a bypass valve 40, and/or a suction modulation
valve 44, both associated with the secondary compressor 28, to
limit the amount of refrigerant compressed by the secondary
compressor 28, and thus to unload the compressor 28. By reducing
the amount of refrigerant compressed by the secondary compressor
28, the speed of the expander 34 mechanically coupled with the
compressor 28 can be increased. The expander speed adjustment
achieves desired thermodynamic characteristics of the expanding
refrigerant that can be optimized for specific operating
conditions. The desired thermodynamic characteristics of the
expanding refrigerant tailored to a specific set of operating
conditions are as known in the art, and have been utilized for
operation and control of electronic expansion valves. However,
achieving desired thermodynamic characteristics of the expanding
refrigerant have been limited with systems utilizing expanders,
since the expander speed is not usually actively controlled.
[0017] However, by utilizing the control 50, and selectively
operating, for example, either the bypass valve 40 to control the
amount of refrigerant bypassed through a bypass line 42, or by
limiting the amount of refrigerant passing through a suction
modulation valve 44 and reaching the secondary compressor 28, the
amount of refrigerant compressed by the secondary compressor 28,
and thus the speed of the expander 34, can be controlled. The
control 50 may also be operated in a pulse width modulation mode to
rapidly cycle either valve 40 or 44 between open and closed
positions to achieve precise control over the amount of refrigerant
compressed by the secondary compressor 28. Obviously, the valves 40
and 44 may operate in conjunction with each other to achieve the
desired level of unloading of the secondary compressor 28.
[0018] Compressor unloading can be accomplished by using various
unloading techniques such as, for example, moving a slide valve of
a screw compressor, opening a bypass port of the scroll compressor,
using suction cutoff of a reciprocating compressor, installing a
suction modulation valve, or utilizing any other known techniques
to reduce the compressor load.
[0019] To be operational and to take advantage of the invention,
the expander 34 does not have to be connected to the high source of
pressure associated with the heat rejection heat exchanger 32 and
to the source of low pressure associated with the evaporator 38. To
perform the expansion function, the expander can be connected to an
intermediate pressure point in the refrigerant system 120 as shown
in FIG. 2. In refrigerant system 120, the main compressor may
consist of two compressor stages 22 and 222 connected in series. In
the embodiment shown in FIG. 2, the expander 34 is incorporated
into a loop associated with a vapor injection or economizer cycle,
where the expander 34 is expanding the refrigerant from the
pressure associated with the heat rejection heat exchanger 32 to
the intermediate cycle pressure approximated by the pressure
between the first compression stage 22 and the second compression
stage 222. Economizer cycles are known in the art, and the benefits
provided by economizer cycles are associated with additional
subcooling obtained in the economizer heat exchanger 224 and a more
efficient compression process, due to refrigerant vapor injection
between sequential compression stages 22 and 222. The refrigerant
undergoing expansion in the expander 34, from a high-side to
intermediate pressure, provides even greater subcooling to the main
flow in the economizer heat exchanger 224, where the main flow
undergoes expansion in a main expansion device 226. This greater
subcooling, and higher cooling thermodynamic potential for
refrigerant entering the evaporator 38, is achieved due to more
efficient isentropic expansion process, in comparison to
isenthalpic expansion process provided by traditional expansion
devices. The expansion device 226 can be, for example, a fixed area
orifice, a capillary tube, a thermostatic expansion valve, an
electronic expansion valve, another expander or a combination of
different expansion devices. As in the embodiment shown in FIG. 1,
the expander 34 of the FIG. 2 embodiment is associated with
secondary compressor 28 and takes advantages of the selective
unloading of this compressor, as discussed above. In this case, the
secondary compressor 28 operates in a parallel arrangement (or in
tandem) with the primary compressor 22, which in combination with
the compressor 28, provide the first stage of compression, from a
suction pressure to an intermediate pressure. Of course, as known
in the art, the two compression stages 22 and 222 may be provided
within a single compressor housing.
[0020] Similarly, in the embodiment 220 shown in FIG. 3, the
secondary compressor 28 may be positioned to operate in parallel
(or in tandem) with the second compression stage 222 and to
compress refrigerant from an intermediate pressure to a discharge
pressure. Other arrangements are also possible, where for instance,
the main and secondary compressor operating in tandem may compress
refrigerant to a pressure lower then the pressure associated with
the heat rejection heat exchanger 32. Further, if multiple
intermediate pressure levels are available within the refrigerant
cycle, the secondary compressor 28 may operate between its own
pressure levels, and not exactly in tandem with any of the primary
compressors. These arrangements would also be typical of
compressors installed in series.
[0021] Even further arrangements are possible, where, for example,
the secondary compressor 28 is not compressing the refrigerant, but
instead is compressing some other process fluid. In this case, in
the embodiment 320 shown in FIG. 4, the secondary compressor may be
used, for example, to compress air and deliver it from an inlet
line 321 to an outlet line 322. As described above, a similar
bypass arrangement may be used to control the amount of the
bypassed air to shed off the compressor load to control the speed
of the expander. Of course, in this case, since both the compressor
28 and the expander 34 are located on the same shaft, a special
seal needs to be added onto the rotating shaft, as known, that
would prevent the leakage of the refrigerant to the ambient
environment.
[0022] Further, in all the embodiments above, a clutch can be
installed on the rotating shaft 36 connecting the secondary
compressor 28 and the expander 34 to selectively engage and
disengage a mechanical coupling of these two expresser
components.
[0023] It should be pointed out that many different compressor and
expander types could be used in this invention. For example,
scroll, screw, rotary, centrifugal or reciprocating compressors and
expanders can be employed.
[0024] The refrigerant systems that utilize this invention can be
used in many different applications, including, but not limited to,
air conditioning systems, heat pump systems, marine container
units, refrigeration truck-trailer units, and supermarket
refrigeration systems.
[0025] Furthermore, it has to be understood that although this
invention can be applied to any economized refrigerant systems, the
refrigerant systems employing CO.sub.2 as a refrigerant would
particularly benefit from this invention, since these systems have
inherit deficiencies and require additional means for the
performance enhancement.
[0026] Although a preferred embodiment of this invention has been
disclosed, a worker of ordinary skill in the art would recommend
that certain modifications would come within the scope of this
invention. For that reason, the following claims should be studied
to determine the true scope and content of this invention.
* * * * *